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presure loss per head

Could someone please explain pressure drop. I understand that if I have 50 psi at the valve, and with 3/4" pvc @5gpm there would be 1.45 psi loss per 100 feet. But say I have 200ft of 3/4pvc with 4 or 5 maxipaws spread, I get the 3psi drop from the pipe, but I dont know how to calculate psi drop from the heads??
Thanks tons

It's not a question of pressure drop at the head. It's a question of do you have enough pressure left by the time the water reaches the heads.

Wet Boots has it right, you need to read the materials available at www.irrigationtutorials.com.

But the basic idea is you start with your static water pressure, subtract off all of the expected pressure losses, and the resulting value is the working pressure of the heads. The specifications of the heads will tell you what pressure ranges it can operate at and what it's performance (usually in the way of water radius and gpm) will be a various working pressures.

So usually, one of the ways to work it is to assume an expected working pressure at the heads (say 40psi). You look at the specs of the sprinkler heads and determine what their gpm flow rate is going to be at that pressure. You then use those flow rates to determine your pressure losses in the meter, filter, backflow, valves and pipe. If your static water pressure minus all these calculated pressure losses is less than the assumed 40psi working pressure at the head, then the head will not perform as designed. You will have to find ways to reduce your pressure losses (larger pipe and lower total gpm by using fewer heads per zone).